In recent years, as output and performance of automobiles are enhanced, tires are also strongly required to enhance the steering stability together with riding comfort.
In a pneumatic tire, it is known that the steering stability can be enhanced by increasing the tire lateral rigidity. Therefore, a reinforcing layer using steel cord or organic fiber cord is conventionally provided over a wide range from a bead portion to a sidewall portion to enhance the flexural rigidity of the sidewall.
However, if such a reinforcing layer is used, the tire vertical rigidity is increased and thus, the riding comfort is deteriorated. Further, stress is prone to be concentrated on an outer end of the reinforcing layer. Especially when the reinforcing layer is used for a high performance tire in which a tire aspect ratio is reduced to 55% or lower and a ground-contact width and a ground-contact area are increased, a flexible region of the sidewall portion becomes narrow and stress is concentrated more. Thus, there is a tendency that the endurance is deteriorated.
Thereupon, the present inventor focused attention on a fact that not only the tire lateral rigidity but also twisting rigidity of the bead portion largely affected the steering stability, and researched. As a result, the inventor found that the tire vertical rigidity could be maintained at low level and the steering stability could be enhanced without deteriorating the riding comfort if a reinforcing layer in which reinforcing cords were arranged at a predetermined angle and which had a folded-back portion was formed around the bead core without projecting the reinforcing layer from a bead apex rubber, and the bead core and the bead apex rubber were formed integral with each other, and the twisting rigidity of the bead portion was enhanced.
The steering stability can be enhanced also by increasing the width of the bead core to increase the twisting rigidity and to enhance the fitting performance between the bead core and a rim. In such a case, however, since the steel amount of the bead core and a rubber amount of the bead apex rubber are increased, there is a problem that the tire weight is increased and the fuel performance is deteriorated.
Based on an idea that a predetermined reinforcing layer is provided around a bead core, it is an object of the present invention to provide a pneumatic tire capable of enhancing the steering stability without deteriorating the riding comfort and without excessively increasing the tire weight.